Sea of Japan, which is known to be the sea between Korea and Japan is only asserted by Japan's perspective. Historically and politically , the sea is the territory under Korea. However, Japan still asserts the sea should be called "Sea of Japan". As you know, Japan is a war-criminal country which is responsible for the World War II. Japan attacked many Asian countries including Korea and killed a lot of people brutally for making the attacked countries their colonies. Korea was the country which got the biggest damage by Japan then.
In the colonial years, Japan killed a lot of my people and took a lot of resources, money, and human dignities. Japan even killed the queen of my country then and tried to take all the territories and exploited all the benefits of my country.
In the process, Japan named the East Sea as Sea of Japan.
Therefore, the name was only used under the colonial year during the World War II and it shouldn't be used now because it is a part of the remained evidence of exploitation under the colonial years by Japan.